Thermal-structural analysis of a Cylinder Head using the Workbench Platform. A unique calculation environment for different vertical codes

Figure 1: Logical flux – from CFD to thermal structural analysis

The design and verification of a cylinder head is usually the most critical application of the structural optimization of the assembly behavior.

Nowadays, complex geometries and different schematization requirements due to CFD and structural issues are causing an infinite number of problems. These technical difficulties are mainly related to different meshing topologies and to the single non-linearity of each numerical code.

This article describes the flexibility of the Workbench Platform, a technical approach that is characterized by its capability to connect the different codes and, at the same time, to point out the single peculiarities of each code.

A CFD analysis using the Vectis code was performed. The results with regard to convection coefficients and bulk temperatures were mapped on a FEM thermal meshing generated in the ANSYS Simulation environment. Then, these results were used for the structural non-linear analysis performed in ANSYS.
At the same time, a pilot project has been developed with the aim to verify the efficiency of the Workbench 12.0 platform as a unified environment. All the technical activities of the project were focused on a fluid dynamic, thermal-structural analysis applied to an automotive cylinder head.

Figure 2: the simulation tree

Figure 3: the customized interface: link to the remapping macros

The analysis has been completely performed with the ANSYS Workbench 12.0 platform. The engineers involved in the work could take advantage of the following Workbench capabilities:

• Multiphysics platform
  
• Tool for integration of different technologies (external fluid dynamic software, ANSYS Thermal, ANSYS Mechanical)
  
• Tool for advanced meshing and solving of models with a high number of degrees of freedom.

When we look at the details of the project work, they can be best described and summarized as follows:
The logical flux of the analysis of a cylinder head which starts with the CFD results of the combustion chamber and the cooling system obtained by the Vectis code, transfers the information from the thermal analysis to the structural pre-stressed non-linear analysis in ANSYS.

The added-value of this project is the customization of the Workbench interface which, in fact, realizes an automatic link to the Vectis results and ensures the possibility of remapping the fluid dynamic results on the thermal-structural mesh. The latter is completely different from the CFD model.

First step: From CFD results to thermal conditions
So far, the traditional working process has been to perform a unique mesh for the CFD and thermal-structural analysis in order to be able to remap the fluid dynamic results, in terms of bulk temperatures and convective coefficients, on the mechanical mesh. Hence, the thermal-structural model had to be created with the same topology as the CFD model which caused long modeling times.
Furthermore, the software used for the fluid dynamic, thermal and structural analysis, in many cases, belonged to different families of software (ANSYS, Nastran, Abaqus, Vectis…).
This scenario caused considerable increases in costs and a number of problems concerning interfaces and other interpolations.
Owing to the newly introduced procedure, it has become possible to interpolate, in an automatic way, the fluid dynamic results on the mechanical mesh which is completely different (from a topological point of view) to the CFD grid. This led to a severe reduction of modeling time and number of obtained elements (from a fine CFD mesh to a coarse thermo-structural model).

Figure 4: thermal-structural coarse mesh-CFD fine mesh

The only constraint for the proper interpolation of the CFD results is to have a proper spatial positioning of the thermal structural model compared to the fluid dynamic model. In other words, the two geometrical models must have the same spatial position. Thanks to the new customization realized with the j-script and ANSYS APDL language, it is now possible to apply the correct fluid dynamic loads simply by taking into account the Vectris results file and by creating a component with those nodes that require the interpolation. In this way, the same model can be used to perform the thermal analysis and the structural pre-stressed analysis, saving time during the preparation and the solution of the model, thus avoiding further phases of interpolation of temperature values for the structural model.

Second step: From thermal analysis to structural analysis
The results of the thermal analysis, performed with bulk temperature and convective coefficient evaluated with the CFD method, are transferred to the structural model without any further interpolation thanks to the same FEM model which can be used for both analyses.

Figure 5: Equivalent Von Mises stress in the last time step

In this specific case, the analysis was carried out in several steps to take into account the pre-heat stress, and to subsequently model both: the phase of bolts clamping and the phase of the valve seat clamping with the head. The last step represents the real loading phase linked to the pressure in the combustion chamber. Both the non-linearity related to material yielding as well as the non-linearity related to contacts are considered.